why put in an older engine?
because thats what the french did to keep it running. its common enough and they only send us old crap via these antique dealers who buy the stuff by the lorry load.
Possibly someone has tried to make the bike look like this from 1929
much better and clear pics needed of both sides; Is the head alloy? I suppose its remotely possible you have a TT engined one,but I doubt it.
Your frame has had a lot of extra bits welded on to it,all those triangular plates and braces for a start.
All in all its an example of why no one who is a beginner with old bikes should never buy this french stuff. Has it got a irish reg?
Ps I found some stuff in a french magazine that might help, I will scan it and post it tomorrow